In the previous chapter we used Feynman diagrams to evaluate Hereditary effects in post-Newtonian theory up to 5PN order. However, the dynamic of a binary coalescing system is non-conservative, since it involves the emission of radiation, and just dictated by initial conditions in time. The use of Feynman diagrams instead is based on symmetric in time boundary conditions, and it is not the correct approach to deal with dissipative problems. As emphasized in [1]}, we should study radiation problems like this using the so-called Schwinger-Keldysh (or In-In or CTP) formalism, which was introduced by Schwinger in [2]}, [3]}, to compute expectation values in quantum mechanics from a path integral formalism.
